The question says everything, knowing that a pdf is protected using standard Adobe password encryption that comes with Acrobat Pro (which as far as i know is AES 128) how much would it take to bruteforce a key which is known to be 20 characters long and that the charset is A-Z, 0-9? Please assume for calculation a modern GPU rig, or a GPU cluster from for axample Amazon AWS.
A-Z and 0-9 means 36 possible characters. 20 such characters imply 3620 possible keys. That's approximately equal to 2103.4.
The biggest brute force effort currently known publicly was for a 64-bit key (for RC5, but the difference between RC5 and AES is not important here); it is described here. It took almost five years and a lot of contributors; the peak cracking rate was equivalent to what 30000 top computers of that time could do. Of course, this was a decade ago, and computers have become faster, but not to the point of closing the gap from 264 to 2103.4: we are talking about a problem which is 725 billion times harder.
GPU would not be a very efficient platform for AES-breaking; the most cost-effective system would be CPU with AES-NI opcodes. Note, though, that an AES key is a sequence of 128, 192 or 256 bits; not a sequence of characters. Therefore, your characters are probably transformed through some kind of hashing into an AES key, and the hash function computation will probably be more expensive than the AES invocation itself. Depending on the used hash function, GPU may become competitive again. In any case, we are talking about, at best, a few billions of keys per second and per GPU. One billion of such GPU would produce a lot of heat... and would still need billions of seconds to get through (one billion of seconds is 30 years).
So the only realistic answer to your question is: forever. An key of the format you describe (20 characters in an alphabet of size 36) will not be cracked through brute force. Brute forcing such a key would not make sense: even if it was technologically feasible (which assumes more resources than is available to the biggest governments or corporations currently existing), it would cost a lot more than whatever the key is protecting. For instance, if I owned the millions of billions of dollars involved in the process, then I would simply buy the USA wholesale (top corporations, government,... including a complete national debt buyout)(that is, if it took my fancy to actually own the USA, which is, when you get down to it, a weird idea).
(In all of the above I am using the American "billion", i.e. one thousand millions, not one million millions.)
Well lets take an average processor speed, not too fancy, cracking approximately at 22,004k/s a PDF document. Assuming you are only bruting a-z0-9A-Z with out spaces, special characters, etc.
It would take approximately 1 septillion years. or 1.0306281275164522e+24 years 33 days 7 hours 30 minutes and 54 seconds
The amount of password combinations you would potentially have to test is (7.159713505559651e+35 password combinations)
Again I am assuming we are NOT using any GPU, Amazons AWS, etc. We are using an average processor. If you use GPU or AWS, this would SIGNIFICANTLY speed up your cracking. You can play with the numbers at this. site.
I hope this answered your question.